Radiomic antennas height vs. RF noise issue

[LadislavKiprich]

Hi, this weekend I had oportunity to play with my RF Explorer, at production of TV show.

My wireless mics setup was simple, just 6 channels of Sennheiser EW500G2. Because it was TV show, it had many LED screens. So when I measured RF spectrum on places where I would commonly place paddles, it showed me a highnoise floor. Back to the rack, about 50cm above the ground the noisefloor dropped down by at least 15dB. So at the end I just used whip antennas, working absolutely fine.

Does anybody else has such experience, and is there a point to use paddles (as theory suggest mounted high), to cut the distance by 4-5 meters between TX and receiver, but sacrifice the signal to noise ratio?

 

Thank you very much for any input.

[GaryNattrass]

Were you using the active sennheiser paddles ? any gain added at the paddle end tends to give more problems than it's worth in some situations and can raise the noise floor a great deal.

 

Using a high gain antenna may work in some situations but sometimes keeping the receivers close with smaller antenna and therefore with less gain can give you more control as you have found.

[paulears]

I suspect it's just the usual RF rule working in this case against you - to get longer distances, get your aerials up high. It's actually working, isn't it - 'you are hearing' emissions from further away. In this case, you don't want them!

 

I've, over the years, become convinced the quest for signal strength is not valid. The aim should be for the minimum of black spots. My thinking is that if you watch the meters, for most of the time, they show full strength, but the nulls are very deep, often dropping to zero when the transmitter is in a particular place. Often, the receive aerials are so close together that both front ends in the receiver are presented with no signal. Moving one of them maybe only two or three metres can present a new path that is solid - so this is all I do. Aerial spacing is so much more effective than pre-amps, paddles and extra height. These things have their uses, of course, when signal strength really is low. usually extra distance, obstacles etc. However, how many times do we actually have dropouts with paths from the wings to centre stage? directional aerials, height, preamps and ultra low loss cable can easily bring in exactly the rubbish being talked about here - I just wonder if it's necessary?

[sleah]

Were you using the active sennheiser paddles ? any gain added at the paddle end tends to give more problems than it's worth in some situations and can raise the noise floor a great deal.

 

Using a high gain antenna may work in some situations but sometimes keeping the receivers close with smaller antenna and therefore with less gain can give you more control as you have found.

 

Interestingly, the Trantec 4.16 UHF system when used with remote attenna, although whip rather than paddle, have a small rf amp as part of the antenna mounting bracket. :unsure:

Not that knowing that helps anyone!

[LadislavKiprich]

Were you using the active sennheiser paddles ? any gain added at the paddle end tends to give more problems than it's worth in some situations and can raise the noise floor a great deal.

 

Using a high gain antenna may work in some situations but sometimes keeping the receivers close with smaller antenna and therefore with less gain can give you more control as you have found.

 

Garry,

before trying the active paddles I used this: http://rfexplorer.com/models/ Even on same spot I was measuring noise floor at about -100dBm at low height and about -85dBm at 2 meters height.

It is very interesting just to walk around and see how the spectrum changes and actualy see, where is the best place for antennas.

 

And yes, sometimes less is more, even with radio waves .

[cedd]

It's interesting that this has come up.

 

I've just been chasing an interference problem at my local venue. We're getting occasional "splats" and poor RF performance. As a friend (the Sound Designer for this show) has just bought an RF Explorer (as will I when my bank account looks more healthy!) I took it for a drive on the last night of the performance to see if I could track the thing down.

 

As it turns out it was a local TV relay on channel 39 - nothing we can do about it - it's perfectly legal and is far bigger than us! We're just going to have to think carefully about aerial type and positioning, and maybe look at some filtering.

 

But what I did discover was a massive difference in noise floor between high ground and low. Nothing new in that, but I'd never seen it for myself and the scale of it shocked me.

 

Apologies for the pictures - camera phone pointing at the RF Explorer, and it was raining! Oh and I also took most of these readings in a hurry - most of the places I stopped were lay-bys and secluded car parks, better known for night time activities, if you catch my drift!

 

http://i513.photobucket.com/albums/t335/ceddison/Yeadon%20Interference/42276E57-0E36-47AD-B1B4-15CF60957C24-18962-00001E7410EFC860_zps6d0817eb.jpg

 

This is the interference I was chasing - the square lump about a quarter of the way across the screen - basically all of channel 39. The chunk to the left of it is of course channel 38 and you can just see a couple of the radio mics poking through, despite being outside the theatre. The lump on the far right is Emley Moor TV mast - A known tv channel - I wanted to compare its' level to our interference - we suspected it may have been from the same mast (it wasn't!) so theory would have said its' level would have tracked that of the interference.

 

http://i513.photobucket.com/albums/t335/ceddison/Yeadon%20Interference/79655E71-3F37-4CCA-94CA-14D02CD22306-18962-00001E74385B5D2F_zps8f5873ed.jpg

 

This is taken in the Town Centre, about a mile down the hill. As you can see, all's well!

 

http://i513.photobucket.com/albums/t335/ceddison/Yeadon%20Interference/363361A8-5FD6-48E2-95B0-78A9EA78F5A2-18962-00001E73AE8B38F4_zps8e06e99d.jpg

 

And this was on the highest bit of ground in the area, with direct line of sight to several TV masts. As you can see, we're getting tonnes of rubbish, including in channel 38 which, in theory, nothing local should be radiating on.

 

The moral of the story - don't build theatres on hills!

 

But as you can see, you're entirely right, elevation does have an impact on the noise floor. This is an extreme example though, with a difference of a few hundred feet. In your case the interference sounds to be pretty locally generated. I'd say though that perhaps it's been your change from paddles to whips, rather than the loss in height, that's helped you out here. Which paddles are they? Directional or omni (no antenna is a true omni, but it's near enough!)? Remember that a directional antenna also has back lobes, so just because it's pointing away from something doesn't necessarily mean it's rejecting it. Remember also that gain in an antenna is as good at amplifying noise as your mics.

 

We really need somebody out there to manufacture some in line filters built for radio mic frequencies. Passive filters are very hard to get "tight" but they'd still be able to remove a lot of the out of band noise, which would help with receiver desense. Something more selective would probably have to be powered though. They do them for the TV world, so I can't see why they can't be made for us too. I seem to remember somebody at Lintec saying they were designing some, but it was a while ago now. The best way of filtering though is to never let it get in to your antenna system in the first place - antenna positioning and choice again! Don't use wideband antennae (like I'm attempting to do!!).

 

Sorry, I realise the bit about my interference search was slightly off topic from interference within a room, but hopefully it was of use and interest!

[GaryNattrass]

LIke all RF you need to do walk tests and place the receivers to suit the coverage rather than trying to do too much and risk pulling in unwanted fringe frequencies although at times it is hard to get anything if the OB next to you is blasting out umpeteen watts of talkback to try and get a signal into a football stadium.

 

For talkback systems I tend to use localised repeaters and at times for radio mics multiple receivers can be a better option than trying to get a huge coverage from larger antennae.

[mackerr]

Hi, this weekend I had oportunity to play with my RF Explorer, at production of TV show.

My wireless mics setup was simple, just 6 channels of Sennheiser EW500G2. Because it was TV show, it had many LED screens. So when I measured RF spectrum on places where I would commonly place paddles, it showed me a highnoise floor. Back to the rack, about 50cm above the ground the noisefloor dropped down by at least 15dB. So at the end I just used whip antennas, working absolutely fine.

Does anybody else has such experience, and is there a point to use paddles (as theory suggest mounted high), to cut the distance by 4-5 meters between TX and receiver, but sacrifice the signal to noise ratio?

 

Thank you very much for any input.

 

Were you using a paddle antenna on the RF Explorer? If not, it wasn't representative of what a paddle would show. Did you use the RF Explorer to measure the RF level from the mic packs, with the mic packs where they will be during the show? While seeing a rise in ambient noise floor might steer you away from an antenna position, the rejection of the paddle may lessen that, and what you really want to know is how much above the noise the antenna picks up are the mic transmitters.

 

If paddles are available I don't see a reason to use omni whips with no passive gain. By positioning the directional antennas so they both reject unwanted noise, and have gain for the mic transmitters you are able to optimize both.

 

Mac

[LadislavKiprich]

Were you using a paddle antenna on the RF Explorer? If not, it wasn't representative of what a paddle would show. Did you use the RF Explorer to measure the RF level from the mic packs, with the mic packs where they will be during the show? While seeing a rise in ambient noise floor might steer you away from an antenna position, the rejection of the paddle may lessen that, and what you really want to know is how much above the noise the antenna picks up are the mic transmitters.

 

If paddles are available I don't see a reason to use omni whips with no passive gain. By positioning the directional antennas so they both reject unwanted noise, and have gain for the mic transmitters you are able to optimize both.

 

Mac

 

No I was using standard antenna. Just after the show I had a thought to make an adapter cable, but I'm not sure about powering the antenna, as it was active - Sennheiser A12AD. I don't remember exactly change of the mic transmitters level, but it was definitely less than noise floor.

 

We really need somebody out there to manufacture some in line filters built for radio mic frequencies. Passive filters are very hard to get "tight" but they'd still be able to remove a lot of the out of band noise, which would help with receiver desense. Something more selective would probably have to be powered though. They do them for the TV world, so I can't see why they can't be made for us too. I seem to remember somebody at Lintec saying they were designing some, but it was a while ago now. The best way of filtering though is to never let it get in to your antenna system in the first place - antenna positioning and choice again! Don't use wideband antennae (like I'm attempting to do!!).

 

The call for filters go little bit against manufacturers trying to get widest tuning range possible. Maybe new digital systems are the answer? We had demo of Sony, my employer will probably buy them. I'm not sure if it is for separate thread, does anybody have experience with it?

 

Sorry, I realise the bit about my interference search was slightly off topic from interference within a room, but hopefully it was of use and interest!

 

You did nice and interesting research.

[TonyMitchell]

I had a situation this week, using 6 channels of Trantec S5.3, lavs on 606.60, 607.500, 608.150, 609.150 and handhelds on 611.250 and 612.300. All fine for the first hour or so, then for no more than 5 minutes loud occasional splats across 607.500, 608.150 and 612.300.

 

At the time this happened, two of the RX packs were with me at FOH, monitoring them gave what I can only describe as a clipped, reverbed version of the FOH PA signal.

 

The location was a school, fairly remote, with the likelihood of other ch.38 use within 200m extremely slim.

 

I was using the whip antennas, via the Trantec ADU, no external paddles. I have never experienced this before and can't emulate it. Most of our jobs are 8 or less UHF channels and for the past two years have been "plug & play". This incident is concerning - as the sound op is left helpless other than to kill the offending channel/s.

 

I'm in two minds now, as to whether to put this down to a "one-off", likely cause being something at the school producing RF... or is it time to invest in a spectrum analyser and monitor the airwaves, rather than assume all clear?

[Brian]

"We really need somebody out there to manufacture some in line filters built for radio mic frequencies."

[mackerr]

I had a situation this week, using 6 channels of Trantec S5.3, lavs on 606.60, 607.500, 608.150, 609.150 and handhelds on 611.250 and 612.300. All fine for the first hour or so, then for no more than 5 minutes loud occasional splats across 607.500, 608.150 and 612.300.

 

At the time this happened, two of the RX packs were with me at FOH, monitoring them gave what I can only describe as a clipped, reverbed version of the FOH PA signal.

 

I don't know if it's time to get a SA to look at the RF environment, but it's really time to get some intermod software to help you choose frequencies. Among your 6 frequencies, 608.150 and 606.600 have an intermod frequency on 611.250, and 611.250 and 612.300 have one on 609.500. If any of those packs were close together they could have been causing the interference you experienced. The symptoms seem to fit that scenario.

 

Mac

[TonyMitchell]

I don't know if it's time to get a SA to look at the RF environment, but it's really time to get some intermod software to help you choose frequencies. Among your 6 frequencies, 608.150 and 606.600 have an intermod frequency on 611.250, and 611.250 and 612.300 have one on 609.500. If any of those packs were close together they could have been causing the interference you experienced. The symptoms seem to fit that scenario.

 

Mac

 

 

Mac - many thanks - I think you've hit the nail on the head.

 

My recollection of the sequence of events is that immediately prior to the Interference, a Q&A runner had given her handheld back to one of the organisers (who was also miked up on a lav) and I'm 90% certain another presenter that had just come off, had given same organiser his pack... hence we had one individual with three transmitters on his person.